The foregoing abstract is not to be taken as limiting the invention of this application, and in order to understand the full nature and extent of the technical disclosure of this application, reference must be made to the accompanying drawings and the following detailed description.
This invention relates to presses for extracting water from a continuous traveling web and particularly to such a press section for extracting water from a newly formed web of paper in a papermaking machine. More particularly it relates to an extended nip press structure of the type including a pressure shoe and a traveling endless belt.
While the present invention relates to dewatering of a continuously running web of any material, it will be described herein with respect to the specific process of dewatering a web of paper. In the papermaking process, the web is formed by depositing the slurry of pulp fibers on a traveling wire. A large portion of the water is normally extracted from the web in the forming area by gravity or suction. The web then passes through what is known as a press section which normally would involve a series of nips of pairs of roll couples in which a substantial amount of the remaining water is squeezed out. The web will then pass on to a drying section which normally is composed of a series of heated drums to drive water off by vaporization. The web then finally passes to such finishing operations as calendering, coating, slitting, winding, et cetera.
The present invention relates specifically to a particular type of press section wherein the pressing operation in each unit is extended in time and thereby results in the extraction of significantly more water than in the heretofore nip of a roll couple. This extended nip pressing is accomplished by wrapping an endless belt about an arc of a rotating drum. The web is sandwiched between the endless belt and the drum and will have a traveling felt on one or both sides thereof for absorbing the water from the web. Additional pressure is provided to the arc of contact area by means of a pressure shoe located on the side of the belt opposite the drum.
The principles and advantages of extended nip pressing have been discussed in U.S. Pat. Nos. 3,798,121 and 3,853,698, both of which are assigned to the assignee of this invention. These principles and advantages, therefore, need not be discussed herein. The present invention, however, is related to an extended nip press of the type disclosed in U.S. Pat. No. 3,853,698 wherein a pressure shoe located on the side of the belt opposite the drum to generate high pressing forces against the web. This is to be distinguished from the type disclosed in aforesaid U.S. Pat. No. 3,798,121 in which the pressure is provided by tension in one or more belts as they pass about the drum.
In the operation of such extended nip press sections having a pressure shoe, a problem has evolved wherein a bulge or bow forms ahead of the nip. The exact phenomenon which causes this bow or bulge is not fully understood. It is clear, however, that the center portion of the endless belt in the area of the shoe is compressed, heated by the oil and friction and is otherwise worked differently than the rather wide edges of the belt. The bulge will sometimes be centered on the belt and at other times will be off to one lateral side of the belt. It will sometimes appear on the downstream side of the shoe on the laterally opposite side of the belt relative to a bulge on the upstream side of the shoe. Experience thus far shows that the bulge is always confined in lateral directions to the shoe area.
Needless to say, this bulge in the belt is undesirable for many reasons, among which is the fact that it can cause wrinkling or creasing of the web. While the bulge can be eliminated by increasing the tension on the belt, this is not fully satisfactory since it causes increased loading on belts, shafts, bearings and drives. This in turn results in a decrease in the service life of such components and an increase in power consumption and down time.
The complexity of the operating conditions renders a solution to the problem evasive. Presently, pressure shoes having a 10 inch (25.4 centimeters) arc of contact and pressures of 600 pounds per square inch (42 kilograms per square centimeter) are utilized in experimental machines. This means that the belt is subjected to 6,000 pounds of normal force for every inch (1071 kilograms per centimeter) of width of the belt in the shoe area. Further, it is contemplated that pressures may be increased to 900 pounds per square inch (63 kilograms per square centimeter) or above, and arcs of contact might be increased to as much as 20 inches (50.8 centimeters) or more. A 20 inch (50.8 centimeter) arc of contact and shoe pressures of 900 psi (63 kilograms per square centimeter) would result in 18,000 pounds of normal force for each inch (3213 kilograms per centimeter) of width of the belt in the shoe area.
Further, since the belt is in sliding contact with the shoe and under extremely high pressure, significant heat can be generated due to the sliding friction. The hydraulic fluid in the shoe is maintained at 140.degree. Fahrenheit (46.degree. Centrigrade) to maintain the proper viscosity. With the heat caused by the sliding friction and hysteresis losses in the belt added to the heat from the oil, it is believed that belt temperatures may approach 200.degree. Fahrenheit (79.degree. Centrigrade).
In my co-pending U.S. Pat. No. 4,229,253, filed Apr. 26, 1979, (assigned to the same assignee as this invention) it is suggested that longitudinally extending cords be provided only in the area of the belt which passes through the pressure shoe area. It is further noted in said co-pending application that by providing such longitudinals cord in the shoe area only, a substantial reduction in the tension required to eliminate the bulge is realized.
In co-pending U.S. Pat. No. 4,229,254, filed Apr. 26, 1979, (assigned to the same assignee as this invention) it is proposed that the longitudinal reinforcing structure be comprised of at least a pair of layers of cords extending respectively at equal but opposite small angles with respect to the longitudinal direction of the belt. In that co-pending application, it is noted that if the cord angle with respect to longitudinal direction is low and the modulus elasticity of the cords is sufficiently high, proper circumferential resistance can be provided and at the same time possible side to side variations and tensions throughout the shoe area can be balanced.
In co-pending U.S. Pat. No. 4,238,287, filed Apr. 26, 1979, it is suggested that a transverse stiffening system be provided which resists the bending necessary to form the bulge ahead of the shoe area.
In accordance with the present invention, yet another method and means of reducing the tension required to draw the bubble or bulge out of the belt is proposed. This concept can be used in conjunction with one or more of the three aforementioned techniques of reducing this required tension or in place of these techniques.
More particularly, the present invention involves the relieving of the lateral edge contact area between the belt and the drum which is disposed laterally outside the pressure shoe area. In the preferred embodiment, a reduced diameter portion is provided in the laterally outer portions of the rotating drum. Alternatively, a reduced thickness or cutaway portion can be provided in the continuous belt in the area corresponding to these portions laterally outside the pressure shoe. Lastly, relieved laterally outer portions can be provided on both the endless belt and the rotating drum.
An object, therefore, of the present invention is to provide relief in a laterally outer portion of the rotating drum and endless belt combination in an extended nip press to reduce the tension required to eliminate bubbles in the belt adjacent the nip of an extended nip press structure.
Other objects, advantages and features will become more apparent with the disclosure of the principles of the invention and it will be apparent that equivalent structures and methods may be employed within the principles and scope of the invention in connection with the description of the preferred embodiment and the teaching of the principles in the specification, claims and drawings.